The floors of the Miraflores and Pedro Miguel locks will have 1 foot thickness of concrete on top of the rock as a wearing surface. At Gatun, however the rock is of a character susceptible to the weather. It has therefore been considered necessary, in constructing the floor here, to leave the rock above grade until just before the concrete is to be placed and then to scrape and wash the surface to be covered. The floor in the lower portion of the upper chamber is to be of concrete 3 feet thick. The rock here is considered thick enough to withstand the pressure from the water-bearing stratum below. Above the middle gate, however, this stratum is too thin, and a floor 13 feet thick of concrete is provided and anchored by rails set in holes and surrounded by concrete. The main floor level will be about 2 feet below the sills, in order that small objects dropped from vessels may be passed without being struck. The sills for the gates are designed as concrete arches in a horizontal plane, 31 feet thick throughout and of 100 feet radius at the extrados. The filling system is designed so that, with all valves opened the chamber can be filled in 8 minutes, but to prevent possible damage to vessels in the lock the maximum rate will probably not be allowed to exceed 3 feet a minute which would correspond to less than 15 minutes for filling. Most of the foregoing discussion is taken from the Engineering Record of February 26, 1910. There has been much criticism of the lock sites, but the engineers now in charge seem to have perfect confidence in their work. During the fiscal year 1908–’09 the work of excavating for the Gatun locks was continued by steam shovels and one 20-inch suction dredge. Material excavated in the dry amounted to 933,546 cubic yards, and that in the wet to 479,950 cubic yards. It was decided to construct curtain walls to stop any underflow; these will extend across the lock under the sill of the emergency dam and downstream outside the walls to the intermediate gates. As an additional precaution to making the concrete floor 13 feet thick as before mentioned a system of sumps under the floor with telltales in the walls will be built. The plant for the construction of the locks is practically installed and ready for work, it being operated entirely by electricity. At the Pedro Miguel locks 715,726 cubic yards were removed in 1908–’09. One lock chamber was completed to grade, but 45,000 cubic yards remain for removal in the other one. At Miraflores work was done the past year with steam shovels and one suction dredge. The total amount excavated was 1,147,527 cubic yards which is one-half of the total estimated quantity. IX. CONSTRUCTION OF THE DAMS